The difference is that you will be more over the front wheel with the shorter TT and longer stem. It's really preference but I would take the shorter tt if your legs fit b/c the 110 stem is not long at all anyway. Depends if you want a long and stable or short and nimble feel. 1cm won't change a ton though.

True, but under the assumption that the rider is relatively experienced. To new comers getting into road cycling, 100mm stems are often considered "LONG", that's why many still buy 70mm, 80mm stems all the time.

When Lance was still "da man", you see lots of Madones with 80mm stems on 60mm spacer towers all the time.

Body proportion and flexibility/riding style etc. all have impact on geometry.

SL58 wrote:

Very few people outside of circus freak body proportions ride a 100mm stem on 55cm frame.

When I bought the 54cm bike it came with 100mm, the 55cm bike also comes with 100mm by default. Usually I would see 56cm bike built from 110mm. This is for the masses.

I have 1 year experience only but found that I wanted a little more reach after thousands of miles, so slowly getting more reach with 110mm.

I know using no spacers and 120mm is the most pro, but I don't think it applies to everyone.I probably can't tell the difference in handling between the two sizes I was asking about at my experience level.

Very few people outside of circus freak body proportions ride a 100mm stem on 55cm frame.

What are you implying?Each person's body is different.

I ride a 54 top tube with a 90mm stem, and typically (per head tube length) nero-zero spacer stack. If I go with a smaller frame, I'll end up with a large spacer stack and too much toe-overlap on the wheel, with the possibility of not having enough seat-tube for the seatpost if it is a compact frame (I have a 73cm seat height from BB). Long legs, short torso.

Each person's body is different. The 'standard' stem supplied on off-the shelf purchases is just a placeholder. Ultimately when a person is fitted properly things, such as stem length and location, will be adjusted/swapped.

Going back a few posts...conversely there are a lot of road cyclists who run daft long stems just because that's what they've been told . Alternatively others see Pro bikes which are usually a little on the small side with long stems, not realising that a) Pros are often lot more flexible and b) paid on results not comfort.

I think we are starting to see a move more towards 100-110mm being the norm but at the end of the day it's what fits YOU that counts, not some preconception.

Comparing 2 frames that seem pretty close is not a trivial matter, if you are trying to have identical setup.

Seat tube angle - every degree of seat tube angle is ~ 1 cm of top tube, if the saddle setback is the same from the BB. That is a 74 deg seat tube will have effectively a 1 cm longer top tube than a 73 deg seat tube.

Head tube angle - this along with fork rake largely affects how the bike handles but also a slacker head tube angle will put the bars slightly higher for all things equal. A rough guide is ~2mm for every degree of head tube angle. This is also true for stem angle, every degree of rise adds ~2mm.

Head tube length - largely governs how the saddle-bar drop is set. It is not just simple math on the spacers. First off, the difference in BB drop - vertical distance from BB center a horizontal line bisecting the wheel axles - has major effect on bar-saddle drop as a bike with a lower BB will require less distance from the axle to the handlebar and therefore less total head tube length (spacers/stem/head tube combo). To truly work this out one needs to know the differences in BB drop, the difference in fork length from axle-crown, the head tube length, and the head tube angle, as well as the stem length, rise, and height.

For the OP, your two bikes sound pretty close in size given that they have the same seat tube angle. If you are using the same saddle/stem/handlebar between the two then I would suggest to setup the saddle in the same spot on both relative to the BB, then measure from the ground to saddle nose and ground to handlebar top to compute the difference in saddle-bar drop. You can always play around with the spacers to get the fit feeling just right, remember handlebars will have a major effect on this and so will the brake hood. For example, Shimiano 7900/6700 hoods have a longer reach than 7800/6600 or SRAM or Campy.

At the end of the day, I am not convinced two bikes have to be identical in setup. I'm sure someone will argue biomechanical adaptation is crucial but I have 5 bikes with different geometry, groups, handlebars, stems, ane even saddles. All of them are set up pretty close in terms of saddle set back, reach, and saddle-bar drop, probably within a 5-8 mm range. I can switch from bike to bike without any issues. The biggest difference is the shifting, as I have Campy and Shimano setups.

Is there really much of a difference? My knees do not hit the bar on the 54.

Honestly I don't think you will notice it. Theoretically you are moving your c-o-g forward by all of a centimeter which is likely to be less than 2%. But you almost certainly do that on a regular basis as you move around the saddle as fatigue cuts in. You would most likely notice a 130mm stem or a 70mm stem but otherwise I really wouldn't worry - just make sure the stack height is still OK.

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